Copper clad laminates (CCL) are key materials of printed circuit boards (PCB) and are widely used in many technical fields. The copper clad laminates include glass fabric for reinforcement, epoxy resin, and copper foil. Please refer to FIG. 1A illustrating a simplest structure of the copper clad laminate. The substrate 10 interposed between the copper foil 11 and 12 is made of the glass fabric and the epoxy resin. The pieces of the cooper foil 11 and 12 for wires are evenly bound to opposite surfaces of the substrate 10. To enhance strength of the sandwich structure of the copper clad laminate, thickness of the substrate 10 should be greater than 0.1 mm. However, to thicken the substrate 10 is inconsistent with the trend toward smaller size in electronic industry. Moreover, to form wires, the copper foil 11 and 12 formed on the opposite surfaces of the substrate 10 should be patterned through procedures of photoresist application, exposure, developing, etching and photoresist removal. A great deal of chemical solution is used in these procedures, especially exposure and developing. The chemical solution is harmful to the environment. Furthermore, material loading, unloading and arrangement for forming the copper clad laminate require manual operation. Therefore, it is difficult to lower production cost and shorten production period, and human error is probably inevitable.
FIG. 1B is a cross-sectional view illustrates a printed circuit board having a multi-layer wiring structure. The copper wires 19 are formed by wet-etching, and side effect such as undercutting occurs at edges of the copper wires 19. For example, the copper wires 19 have round or sloping sidewalls. The distance of the undercutting is called bias d1. For high power applications, thicker copper wires are adopted to endure high currents. However, thicker copper wires result in worse undercutting and greater bias d1. For example, if the thickness d4 of the copper wires 19 is about 0.15 mm, the bias d1 is greater than 0.15 mm. Please focus on two adjacent copper wires 191 and 192 in FIG. 1B, the narrowest portion and the widest portion thereof are located at a top surface and a bottom surface, respectively. The top spacing d2 is a distance between the top edges (the narrowest portion of the wires in this example) of the two adjacent copper wires 191 and 192, while the bottom spacing d3 is a distance between the bottom edges (the widest portion of the wires in this example) of the two adjacent copper wires 191 and 192, that is, the top spacing d2 is greater than the bottom spacing d3 in this example. For a wire with an irregular cross-section, insulating properties are determined based on a smaller one of the top spacing d2 and the bottom spacing d3. Considering the undercutting phenomenon, the spacing defined in the pattern mask should be increased to maintain required insulating properties. Therefore, the size of the printed circuit board is enlarged while a certain area is wasted.
Accordingly, a multi-layer wiring structure and a related manufacturing method which can overcome the undercutting problems are desired. Furthermore, the multi-layer wiring structure and the manufacturing method can precisely position/align the metal wires and control dimension of the metal wires.